The present invention relates to a system for a four-wheel drive vehicle for automatically changing the transmission system of the vehicle from a two-wheel drive system to a four-wheel drive system or vice versa in accordance with driving conditions.
In a well-known four-wheel drive vehicle, a power transmission system for the two-wheel driving is selectively converted to the four-wheel drive system by engaging a clutch which is manually operated by a select lever.
Such a four-wheel drive vehicle is preferably driven by the two-wheel drive system on paved dry surfaces. If all four wheels are driven on such a dry surface, an amount of tire scrapping occurs because of slight differences in effective wheel radii caused by inevitable differences in tire inflation, tread wear or variation in an loading. This will result in increase of tire wear and fuel consumption and the generation of vibration of the vehicle body and noise. In addition, when the vehicle negotiates sharp corners, braking phenomenon called "tight corner braking" will occur. This is caused by the reason that the front wheels run through an arc of greater radius than that of the rear wheels and therefore tend to rotate faster than the rear wheels.
On the other hand, if the vehicle travels on slippery roads or icy or snowy roads during two-wheel driving, the slipping rate of each wheel increases, because of a decrease of traction of each driving tire and an increase of travelling friction on snowy roads. Therefore, an increase of fuel consumption and, a decrease of driveability and braking performance will occur.